The rules of civil procedure or the code of civil procedure in state and federal courts usually permit a party to a civil lawsuit (a litigant) to amend its pleadings.

A plaintiff can amend the complaint or petition to assert new claims or causes of action before or during trial, subject to certain limitations—and the defendant can amend the answer to the lawsuit to assert new defenses to the plaintiff's claims before or during trial, subject to certain limitations.

In Virginia, the rules governing the amendment of pleadings in civil lawsuits are outlined in the Virginia Rules of Civil Procedure. Generally, a party is allowed to amend its pleadings, which includes a plaintiff amending a complaint or a defendant amending an answer. The right to amend is subject to certain limitations, such as time constraints and the potential prejudice to the opposing party. Amendments may be made before trial or, with the court's permission, during trial. The court has discretion to permit or deny amendments and will consider factors such as the reason for the amendment and whether it would unfairly disadvantage the other party. At the federal level, the Federal Rules of Civil Procedure apply, and Rule 15 specifically addresses amendments to pleadings. Under Rule 15(a), a party may amend its pleading once as a matter of course within 21 days after serving it, or if the pleading is one to which a responsive pleading is required, 21 days after service of a responsive pleading or 21 days after service of a motion under Rule 12(b), (e), or (f), whichever is earlier. Beyond this, parties must seek consent from the opposing party or leave from the court, which should be freely given when justice so requires. Both state and federal rules aim to ensure that cases are decided on their merits rather than on procedural technicalities, while also protecting parties from undue delays and prejudice.